The present disclosure relates to a thermal probe for a near-field thermal microscope and a method for generating a thermal map.
Near-field heat transfer is related to local temperature and thermal conductivity of a sample interface and can be measured e.g. using a scanning thermal microscope (SThM) with a thermal probe comprising a probe tip. Generally, when two bodies at different temperatures are separated by a gap, they can exchange heat via thermal radiation. In the far-field approximation, i.e. when the gap is much larger than the characteristic thermal wavelength, the magnitude of heat transfer may be described by the classic Stefan Boltzmann law. However, when the thermal probe is brought close to the sample interface, i.e. in the “near-field”, non-classical phenomena such as wave interference, surface resonances and photon tunneling can become important, typically causing the radiative heat transfer to increase with decreasing gap size.
In a near-field heat exchange microscope, the distance between probe tip apex and sample interface is comparable or less than the characteristic (thermal) wavelength “λc” e.g. given by Wien's displacement law as
      λ    c    ≅            2900      ⁢                          ⁢      µ      ⁢                          ⁢              m        ·        K                    T      ⁢                          ⁢      K      wherein “T” is the absolute temperature of the sample interface and/or probe tip expressed in Kelvin (K). For the present application, at a temperature above 290 K, a thermal microscope with a probe tip designed to be positioned at or below 10 μm from the sample interface can be considered a “near-field” thermal microscope. For lower temperatures, the near field region can be larger. For example, at a temperature of 300 K, separations below 10 μm typically result in a steeply increasing heat flux with decreasing separation as described by Ottens et al. in Phys. Rev. Lett. 107, 014301 (2011).
McCauley et al. (Phys. Rev. B 85, 165104, 2012) describe modeling near-field radiative heat transfer from sharp objects using a general three-dimensional numerical scattering technique. Spatially resolved heat flux profiles are calculated for different geometries of the object. Cones in particular are found to have a flux pattern exhibiting an unusual feature: a local minimum in the heat flux directly below the tip. This is considered to have important implications for near-field thermal writing and surface roughness where the many tiplike features in a roughened surface could lead to results that differ qualitatively from polarization approximation prediction.
In view of these and other considerations, there is a need for an improved thermal probe and method for generating a thermal map addressing the problems and unusual behaviour encountered in near field thermal microscopy.